1. Field of the Invention
The present invention relates to improved lightweight well cementing compositions having reduced transition times and methods of using the compositions for cementing in subterranean zones penetrated by well bores.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in the construction and repair of oil and gas wells. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing or liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. Hydraulic cement compositions are also commonly used to plug lost circulation and other undesirable fluid inflow and outflow zones in wells, to plug cracks and holes in pipe strings cemented therein and to accomplish other required remedial well operations.
In carrying out primary cementing as well as remedial cementing operations in well bores, the cement slurries utilized are often lightweight to help prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the well bore. As a result, a variety of lightweight cement slurries have heretofore been developed and used including foamed cement slurries. In addition to being lightweight, a foamed cement slurry contains compressed gas that improves the ability of the slurry to maintain pressure and prevent the flow of formation fluids into and through the slurry during the time in which the cement slurry changes from a fluid to a hard set mass. Foamed cement slurries are also advantageous because they have low fluid loss properties.
Well completions in water over 1000 feet deep often require special techniques to install conductor casing. Well completions at depths in excess of 2,000 feet of water are often referred to as xe2x80x9cdeepwaterxe2x80x9d operations. In deepwater operations, the formations where conductor pipe is cemented that is less than 2,000 feet below mud line (BML) are generally young geologically and are not well consolidated. The formations generally are the product of erosion from the continental shelf. This can cause either of two problems in cementing. The formation may be so weak that it may fracture during cementing and cause the loss of cement into the formation. Alternatively, the formation may experience high saltwater or other fluid flow through the formation resulting in fluid influx.
In a typical conductor pipe installation, a 24-30 OD inch surface pipe is driven at least 200 feet BML. A large diameter (20 in. OD when 30 in. OD used) conductor pipe then is cemented by the conventional innerstring method through the drill pipe, with cement returns back to the ocean floor. Since no riser is used, the annular returns must be taken at the sea floor. Cool temperatures caused by the seawater typically slow the cement hydration process and extend the transition time of the cement slurry that allows fluid influx to begin.
The term xe2x80x9czero gel timexe2x80x9d refers to the period of time between when the cement slurry is placed and the gel strength increases to a level of about 100 lbf/100 ft2. During the zero gel time, a fluid such as oil, gas or water is prevented from migrating through the setting cement because the cement column in the well bore can support itself and exert hydrostatic pressure on the fluid surrounding the well bore.
The term xe2x80x9ctransition timexe2x80x9d refers to the period of time it takes the gel strength to increase from a level of about 100 lbf/100 ft2 to a level of about 500 lbf/100 ft2. Fluid migration is substantially prevented at the level of about 500 lbf/100 ft2. During the transition time, a fluid such as oil, gas or water can migrate through the setting cement slurry forming channels that effect the integrity of the cement sheath. The fluid migration is possible during the transition time because the cement column in the well bore begins to support itself and stops exerting hydrostatic pressure on the fluid surrounding the well bore. When the exerted hydrostatic pressure falls below the formation fluid pressure, migration can occur and will continue until the cement develops sufficient compressive strength to prevent further migration. Therefore, it is beneficial to have cement compositions with reduced transition times to help minimize the chances of fluid migration.
In some instances, the formation sands may be over-pressured by water so that water or other formation fluids flows into the setting cement sheath during the transition time. Prevention of such flow is critical to a successful cementing job and to avoid expensive remedial squeeze cementing treatments. Containment of the over-pressured formation fluid often is complicated by weak zones in the formation that can fracture due to the fluid pressure of the cement slurry. If a fracture is formed, the cement slurry can flow into the fracture and be lost from the well bore.
One method for cementing conductor pipes in well bores in water depths greater than 1000 feet is described in U.S. Pat. No. 5,484,019, wherein a cement slurry comprising Portland cement, a foaming surfactant, a quantity of fine particle size cementitious material and nitrogen or other gas to foam the slurry to yield a density of from about 9.0 to about 14 lbs/gal is introduced down the conductor pipe and permitted to return up the annulus of the well bore to the seafloor. The slurry contains calcium chloride (CaCl2) as an accelerator and develops a static gel strength in excess of about 500 lbf/100 ft2 within less than about 30 minutes after placement.
Although the above-described compositions are effective in cementing wells, there is a continuing need for improved lightweight cement compositions that have reduced transitions times to help protect against fluid migration.
The present invention provides improved lightweight well cement compositions and methods of using the cement compositions that meet the needs described above and provide improvements over the prior art. A preferred lightweight cement composition is basically comprised of a hydraulic cement, an effective amount of an iron salt selected from the group of ferric chloride, ferrous chloride or mixtures thereof to reduce the transition time of the composition, sufficient water to form a pumpable slurry, an effective amount of a foaming additive for producing a foamed slurry, and sufficient gas to foam the slurry.
A preferred method of the present invention for cementing in a subterranean zone penetrated by a well using a lightweight cement is accomplished by preparing a pumpable lightweight cement slurry having a density of less than about 14 lbs/gal and an effective amount of an iron compound to reduce the transition time thereof, placing the slurry in the subterranean zone to be cemented, and allowing the slurry to set into a hard impermeable mass. The methods are particularly suited for cementing conductor pipe in a well bore wherein the pipe is situated in the well bore penetrating a subterranean formation so as to define an annular space between the pipe and the wall of the well bore and the well bore is drilled from a seafloor at a water depth of at least 1000 feet. Preferred cement compositions of the present invention are formulated to provide a transition time of less than about 25 minutes, more preferably, less than about 20 minutes and, most preferably, less than about 15 minutes.
It is, therefore, a general object of the present invention to provide improved lightweight cement compositions having reduced transition times for use in oil and gas well cementing applications.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading of the description of preferred embodiments which follows.